Method and apparatus for resonant sensing device

Abstract

A resonant sensing device for measuring the hardness of a workpiece to be tested includes a diamond tipped rod adapted to be resonant and for determining the hardness of a workpiece to be tested is brought into forced contact with such workpiece. The difference in resonant frequency of the rod between its not-constrained condition and its constrained condition (forced contact) is a measure of the hardness of the workpiece. The present invention describes a circuit wherein during a first time interval an up/down counter accumulates the counts arising from the oscillations of the not-constrained rod. During the ensuing interval of the same duration, the counter counts down the accumulated count responsive to the oscillations of the constrained rod. As the counter reaches the zero count condition, which condition is reached before the expiration of the time interval, the counter becomes coupled to an adjustable frequency calibration oscillator and for the remainder of the interval the counter accumulates counts from the oscillations of the calibration oscillator. At the end of the interval, the accumulated count is a measure of the hardness of the workpiece.

What is claimed is: 
     
       1. A method for testing the hardness of a workpiece by the contact impedance method using an oscillating rod excited at its natural resonant frequency wherein the oscillations of the not-constrained rod are counted during a first time interval and the oscillations of the constrained rod, when in forced contact with the workpiece, are counted during a second time interval having the same duration as said first interval and the difference in counts is converted to a corresponding hardness value, the improvement comprising: counting the oscillations of said rod when coupled to the workpiece beginning with the start of said second time interval only until the difference between the count during said second time interval and the count of the oscillations of the not-constrained rod during said first time interval becomes zero, and   during the remainder of said second time interval counting the number of oscillations obtained from an adjustable frequency oscillator.   
     
     
       2. A method for testing the hardness of a workpiece as set forth in claim 1 and including adjusting the frequency of said adjustable frequency oscillator in such a manner that the count accumulated during the remainder of said second time interval is commensurate with the hardness of the workpiece. 
     
     
       3. A circuit for testing the hardness of a workpiece by the contact impedance method using a rod adapted to be rendered resonant, the difference between the resonant frequency of the rod when not constrained and when constrained in forced contact with the workpiece whose hardness is to be measured being a measure of the hardness, including an oscillator for causing said rod to be resonant and an adjustable measuring gate oscillator, both being coupled to an up/down counter, the improvement comprising: means coupling said oscillator causing said rod to be resonant by means of a first input of a switching means to a first input of an AND gate whose output is coupled to said up/down counter;   a measuring gate oscillator having a fixedly adjusted frequency coupled to the second input of said AND gate;   an adjustable frequency calibration oscillator coupled to the second input of said switching means;   a flip-flop coupled to said measuring gate oscillator for being set to its normal state by means of a first input and for being set to its actuated state by means of a second input responsive to a predetermined zero count condition of said counter, and   means coupling the output of said flip-flop to said switching means and to said adjustable frequency calibration oscillator for causing responsive to said predetermined zero count condition of said counter said flip-flop to change its condition from a normalized condition to an operative condition and in response thereto change the condition of said switching means to cause said counter to accumulate frequency counts from said calibration oscillator for a remaining time interval determined by said measuring gate oscillator.   
     
     
       4. A circuit for testing the hardness of a workpiece as set forth in claim 3 and including a frequency multiplier coupled in series between said oscillator rendering said rod resonant and said switching means. 
     
     
       5. A circuit for testing the hardness of a workpiece as set forth in claim 4, said measuring gate oscillator being a quartz-stabilized oscillator and including a binary frequency divider coupled between said quartz stabilized oscillator and said AND gate.